MDA5 is a conserved innate immune receptor that recognizes viral double-stranded RNA (dsRNA) and induces type I interferon immune response. While an appropriate function of MDA5 is important for effective immune defense against viral infection, our recent study showed that its dysregulated activity via gain-of-function (GOF) mutations can lead to a severe inflammatory disease, Aicardi-Goutire Syndrome (AGS) (Rice et al, Nat Genetics, 2014). The goal of this proposal is to further elucidate the molecular mechanisms by which these mutations cause chronic inflammation and AGS. In particular, we focus on defining the potential relationship between MDA5 and a dsRNA modifying enzyme, ADAR1, of which loss-of-function (LOF) mutation has been also shown to cause AGS. We hypothesize that MDA5 and ADAR1 represent two balancing arms of the immune-tolerance relationship. That is, cellular dsRNAs, such as inverted Alu elements, are normally prevented from stimulating MDA5 through modification of adenosine (A) to inosine (I) by ADAR1, whereas LOF mutations in ADAR1 or GOF mutations in MDA5 may allow stimulation of MDA5 by these endogenous dsRNAs. To test these hypotheses and further expand our understanding of the AGS pathogenesis, we here propose a multi-disciplinary research project involving a combination of biochemistry, cell biology and computational modeling. We will first determine the impact of A-to-I modifications on the MDA5:dsRNA interaction using a series of biochemical assays that we have developed in our previous mechanistic studies on MDA5 (Aim 1A). To obtain more detailed structural insights, we will complement this biochemical analysis with computational modeling based on the crystal structure of the MDA5:dsRNA complex that we have determined in the past (Aim 1B). We will next identify the source of the endogenous stimulatory dsRNA for both wild-type and GOF mutant MDA5 using cells derived from AGS patients (Aims 2A-B). This will be performed in close collaboration with Dr. Luigi Notarangelo at Boston Children's Hospital, an expert in the field of inherited immune disorders and has generated and characterized induced pluripotent stem cell lines from patients. This project builds upon our previous work on the structural and biochemical mechanisms of MDA5 and the preliminary data that support our hypothesis. We believe that the proposed research would provide novel insights into the pathogenesis of AGS and other related inflammatory diseases, and also help us define a new paradigm of the complex interplays between the pathogen sensing mechanism and cellular RNA modification.